Assembly for and method of forming localized surface wrinkles

ABSTRACT

An assembly including and method of forming arbitrary localized wrinkles upon a surface utilizing a shape memory polymer substrate and rigid overlay, wherein the geometrical distribution of the wrinkles is produced by recovering a lateral strain history within the substrate and buckling the overlay, and the localized wrinkles are used to create, among other things, optically three-dimensional engaging surfaces, structural colors, modified surface texturing, and haptic alerts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure generally relates to assemblies for and methods of producing localized surface wrinkles, and more particularly, to an assembly for and method of producing localized surface wrinkles using a shape memory polymer substrate and rigid overlay.

2. Discussion of Prior Art

Surface wrinkles have been used to effect, modify, or control various benefits/conditions, including surface adhesion, texturing, coefficients of friction, structural colors, metrology, and haptic alerts. Methods of producing surface wrinkles preexisting in the art include using a stretched substrate overlaid by a rigid (e.g., metal) film. Wrinkles are instantaneously or selectively produced in the film, upon the release of energy by the substrate, if the compressive strain in the film exceeds the critical bucking strain. As a result, these conventional methods produce generalized wrinkles that co-extend with the entire surface defined by the overlay. This method is in fact behind wrinkles commonly encountered, for example, on human skin and dehydrated apples. Of particular interest is that the wrinkle geometry is closely related to the material properties. Precisely controlled wrinkle structures have found many interesting applications including nano-metrology, stretchable electronics, biosensors, and manipulation of material topographic properties.

BRIEF SUMMARY OF THE INVENTION

The present invention recites a novel assembly for and method of producing localized wrinkles within a surface, and more specifically, to an assembly for and method of producing localized surface wrinkles using a shape memory polymer substrate and rigid overlay. The present invention is useful for modifying the surface texture, and/or coefficient of friction of a select portion of a continuous surface. Where achieving a visible wavelength, the inventive wrinkling is also useful for producing structural colors in a predetermined pattern within the continuous surface; and as such, is further useful to produce a three-dimensional engaging surface (e.g., indicia, logo, shape, or picture) on a two-dimensional surface.

In a first aspect of the invention, a method of forming localized wrinkles upon a surface is presented. The method includes indenting a substrate at least partly formed of shape memory polymer presenting a glass transition temperature, so that the substrate defines a first area experiencing purely compressive strain and a second area adjacent the first area and experiencing both a vertical compressive strain and a lateral tensile strain. Next, a overlay is attached to the substrate, so as to overlay the first and second areas. The overlay defines the surface. The polymer is then activated, so as to recover the strains, and cause wrinkles to form in the overlay. Finally, the polymer is deactivated, so as to lock in the wrinkles.

A second aspect of the invention, includes a method of determining the cracking strain/stress of a nanoscopicallythin overlay, which includes observing the wrinkles in the above process, so as to determine a crack formation occurred for a given tensile strain.

Thus, in a third aspect, the invention presents an assembly for forming localized wrinkles within a continuous surface. The assembly includes a substrate at least partially formed of a shape memory polymer presenting a glass transition temperature and first elastic modulus when activated. The substrate is indented so as to define a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing both a vertical compressive strain and a lateral tensile strain, when the polymer is deactivated. The substrate is operable to recover the strains when the polymer is activated. The assembly further includes a relaxed overlay fixedly attached to and configured to cover the first and second areas. The overlay defines the surface, a second elastic modulus, and a height, wherein the second elastic modulus is greater than the first modulus. The moduli, height, and strains are cooperatively configured to cause buckling in the overlay, when the polymer is activated and recovers the strains.

The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

A preferred embodiment(s) of the invention is described in detail below with reference to the attached drawing figures of exemplary scale, wherein:

FIGS. 1 a-d is a multi-elevation progression showing a method of forming localized wrinkles upon a surface, wherein a protruding press is used to indent the substrate, and the wrinkles are further shown in enlarged caption at FIG. 1 d, in accordance with a preferred embodiment of the invention;

FIG. 2 a-c is a multi-elevation progression showing a method of forming localized wrinkles upon a surface, wherein a recessed press is used to form a projection upon the substrate, in accordance with a preferred embodiment of the invention;

FIG. 3 is a perspective view of the wrinkles formed in FIGS. 1 d, and 3 d, in accordance with a preferred embodiment of the invention; and

FIG. 4 is a plan view of a surface presenting a plurality of wrinkles and cracks formed using the inventive method.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. As described and illustrated herein, a novel assembly 10 for and method of forming arbitrary localized wrinkles (i.e., wrinkle structures) 10 a within a surface 12 includes and utilizes, respectively, a locally and plastically deformed shape memory polymer (SMP) based substrate 14 and a thin, high modulus overlay 16 (FIGS. 1-4); however, it is certainly within the ambit of the invention to utilize the benefits of the assembly 10 with other equivalent selectively activated active materials exhibiting shape memory effect, and/or in other applications and configurations discernable by those of ordinary skill in the art.

As used herein, the term “shape memory polymer (SMP)” shall generally refer to a group of polymeric materials that demonstrate the ability to return to some previously defined shape when subjected to an appropriate thermal stimulus, as is known in the art. Shape memory polymers are capable of undergoing phase transitions in which their shape is altered as a function of temperature. The previously defined or permanent shape can be set by curing for thermoset polymers or melting or processing the polymer at a temperature higher than the highest thermal transition for thermoplastic polymers. For a thermoplastic shape memory polymer, a temporary shape can be set by heating the material to a temperature higher than T_(g) or T_(m) of the soft segment, but lower than the T_(g) or melting point of the hard segment. For a thermoset shape memory polymer, a temporary shape can be set by heating the material to a temperature higher than T_(g) or T_(m). The temporary shape is set by cooling. The material can be reverted back to the permanent shape by heating the material above the shape memory transition temperature.

The temperature needed for permanent shape recovery can be set at any temperature between about −63° C. and about 120° C. or above. Engineering the composition and structure of the polymer itself can allow for the choice of a particular temperature for a desired application. A preferred temperature for shape recovery is greater than or equal to about −30° C., more preferably greater than or equal to about 0° C., and most preferably a temperature greater than or equal to about 50° C. Also, a preferred temperature for shape recovery is less than or equal to about 150° C., and most preferably less than or equal to about 150° C. and greater than or equal to about 80° C.

Suitable shape memory polymers include thermoplastics, thermosets, interpenetrating networks, semi-interpenetrating networks, or mixed networks. The polymers can be a single polymer or a blend of polymers. The polymers can be linear or branched thermoplastic elastomers with side chains or dendritic structural elements. Suitable polymer components to form a shape memory polymer include, but are not limited to, polyolefins, epoxy polymers, polyphosphazenes, poly(vinyl alcohols), polyamides, polyester amides, poly(amino acid)s, polyanhydrides, polycarbonates, polyacrylates, polyalkylenes, polyacrylamides, polyalkylene glycols, polyalkylene oxides, polyalkylene terephthalates, polyortho esters, polyvinyl ethers, polyvinyl esters, polyvinyl halides, polyesters, polylactides, polyglycolides, polysiloxanes, polyurethanes, polyethers, polyether amides, polyether esters, and copolymers thereof. Examples of suitable polyacrylates include poly(methyl methacrylate), poly(ethyl methacrylate), ply(butyl methacrylate), poly(isobutyl methacrylate), poly(hexyl methacrylate), poly(isodecyl methacrylate), poly(lauryl methacrylate), poly(phenyl methacrylate), poly(methyl acrylate), poly(isopropyl acrylate), poly(isobutyl acrylate) and poly(octadecyl acrylate). Examples of other suitable polymers include polystyrene, polypropylene, polyvinyl phenol, polyvinylpyrrolidone, chlorinated polybutylene, poly(octadecyl vinyl ether) ethylene vinyl acetate, polyethylene, poly(ethylene oxide)-poly(ethylene terephthalate), polyethylene/nylon (graft copolymer), polycaprolactones-polyamide (block copolymer), poly(caprolactone) dimethacrylate-n-butyl acrylate, poly(norbornyl-polyhedral oligomeric silsequioxane), polyvinylchloride, urethane/butadiene copolymers, polyurethane block copolymers, styrene-butadiene-styrene block copolymers, and the like.

In the present invention, pre-patterning on the substrate overlay 16 is produced to create wrinkle structures 10 a (FIGS. 1 d, 2 c, 3, and 4). The method is based on the principle that the compressive strain on the surface 12 can be altered by indentation or pressing. Depending on the shape and dimension of the indenter or press, the resulting wrinkle structures 10 a can be manipulated in a local and arbitrary fashion.

More particularly, in an exemplary embodiment, the substrate 14 was at least partially formed of a solid epoxy shape memory polymer consisting, for example, of an aromatic diepoxide (EPON 826, 3.6 g or 0.01 mol), an aliphatic diepoxide (NGDE, 2.16 g or 0.01 mol), and an aliphatic diamine curing agent (Jeffamine D-230, 2.3 g or 0.01 mol). This mixture was cured at 100° C. for 1 hour and at 130° C. for 1 hour to obtain a shape memory polymer presenting a glass transition temperature of approximately 40° C., and a permanent default shape. The default shape preferably defines smooth exterior surfacing (i.e., curved or flat but having no indentations or protrusions). The substrate 14 may be rectangular (FIGS. 1-2 c), oblong, define a molding, such as an auto trim, or be of any shape, so long as it is large enough to support a surface 12 suitable for displaying the intended wrinkle structure 10 a. The substrate 14 may include other components, in addition to a body of SMP, such as an external interface layer (not shown) that facilitates bonding with the overlay 16, or non-active sectors where wrinkles are not desired, for example, to better withstand purely compressive forces.

To effect the inventive method, a press (e.g., indenter) 18 with a protruded (FIGS. 1 a-d) or recessed (FIGS. 3 a-d) defining surface 18 a that defines a relief (e.g., three-dimensional logo, indicia, shape, image, or picture) is pressed either manually or automatically onto the shape memory polymer substrate 14 until deformation results (FIG. 1 b, 2 b). More preferably, the press 18 is applied after the substrate 14 has been preheated, e.g., at 65° C. for 10 min in the exemplary embodiment, to a temperature above its glass transition temperature, so as to reduce the modulus of elasticity of the substrate 14, thereby making it easier to indent and deform. The relief can be of any arbitrary shape and dimensions.

In a first directly indented area 14 a, only a compressive strain in the vertical direction is created (FIGS. 1 b, 2 b). On the other hand, no strain is induced in areas sufficiently away from the indented area 14 a. Owing to the material continuity, a second transition area 14 b around the edge of the indented area 14 a is produced. In the transition area 14 b, the strain consists of a compressive component in the vertical direction and a tensile component in the lateral direction. It is appreciated that the spatial distribution of the lateral strain in the transition area 14 b is highly dependent on the shape of the press 18. As such, the resulting wrinkle structures 10 a can be manipulated by alternating the shape of the press 18. The substrate 14 is then cooled back to a temperature below its transition temperature under the load, so as to lock in the deformation.

Next, the relatively rigid overlay 16 is securely attached to the substrate 14, so as to cover the first and second areas 14 a,b (FIG. 1 c, 2 c). In the exemplary embodiment, the deformed substrate 14 is coated at room temperature with a “white gold” film (e.g., palladium/gold alloy composition) 16 using a sputtering system (not shown). Here, the film 16 thickness (e.g., approximately 10 nm) is controlled by deposition time and may be measured directly by a scanning electron microscopic analysis of the cross-sections.

After film deposition is complete, and when the formation of wrinkles are desired, the assembly 10 is heated to activate the polymer substrate 14 and create wrinkles 10 a due to shape recovery induced local compression (FIGS. 1 d, 2 c, 3, and 4). As such, it is certainly appreciated that the overlay (e.g., film) 16 must be non-reactive at temperatures at least equal to the transition temperature of the SMP. In the exemplary embodiment, the polymer may be heated to 90° C. for 10 minutes to ensure complete activation and shape recovery. In the direct indented area 14 a, the shape recovery simply moves the thin film 16 upwards in the vertical direction and induces no strain therein. Again, in the transition area 14 a compressive strain is recovered as the thin film 16 moves upwards. Here, however, tensile strain is also recovered, which creates lateral compression in the thin film 16.

If the lateral compression strain exceeds a critical buckling value defined by the assembly 10, wrinkles 10 a will form. In a preferred embodiment, the critical buckling strain, ε_(c), may be pre-determined according to the following formula:

ε_(c)=[9E _(s) ²/64E _(f) ²]^(1/3)  (1)

wherein E_(s) is the modulus of the substrate, and E_(f) is the modulus of the film; and accordingly the resultant wrinkle amplitude, A, may be determined by the following formula:

A=h[(ε/ε_(c))−1]^(1/2)  (2)

wherein ε is the strain currently experienced by, and h is the thickness of the overlay 16. Thus, it is appreciated that for rigid substrates, i.e., large E_(s) critical strain is large, amplitude is small, and wrinkles are difficult to form. Once the wrinkles 10 a are formed, the substrate 14 is again cooled to a temperature below the transition temperature of the SMP, so as to lock in the wrinkles 10 a, which makes them more robust than those conventionally produced by soft substrate assemblies.

It is appreciated that circularly distributed wrinkles (FIG. 4) are created with a spherical press defining surface 18 a, square-shaped wrinkles would be created by a square shaped press defining surface (FIGS. 1-2 c), and a logo would be created by a logo-shaped defining surface 18 a. By contrast, the wrinkles 10 a may be generated using a Vickers indenter and the first indentation step may be conducted on a non-preheated shape memory polymer substrate 14 using a Nano Scratch Tester (CSM Instruments) under a predetermined load.

In the exemplary embodiment, the wrinkles 10 a were analyzed using an atomic force microsopy (AFM) due to the microscopic scales resulting therefrom. AFM characterization of wrinkles was conducted at room temperature in a contact mode using Dimension 3100 manufactured by Veeco™. The wavelength, a, and amplitude or height, A, of the wrinkles 10 a were obtained by measuring 80-100 individual wrinkles using the section analysis function in the Nanoscope software (Nanoscope 5.31r1). One sample, presented a wavelength and amplitude of 800 nm of 80 nm, respectively.

It is appreciated that the wrinkle wavelength decreases linearly with strain, whereas wrinkle amplitude is independent of strain. Increasing the overlay thickness on the other hand, increases both wrinkle wavelength and amplitude. With respect to the impact of strain, the classical wrinkle theory based on elastic energy minimization suggests that wrinkle wavelength should be strain independent according to the following formula:

$\begin{matrix} {\lambda = {2\pi \; {h\left\lbrack \frac{\left( {1 - v_{s}^{2}} \right)E_{f}}{\left( {1 - v_{f}^{2}} \right)E_{s}} \right\rbrack}^{1/3}}} & (3) \end{matrix}$

where E, v, h, and ε represent respectively modulus, Poisson ratio, film thickness, and compressive strain, and the subscripts s and f denotes substrate and film (i.e., overlay). Finally, it is appreciated that the linear dependence between wavelength and strain under the present invention provides a benefit in creating localized wrinkles 10 a of different wavelength on the same surface, but deviates from the above relationship, when finite deformation is considered.

Where the wavelength falls within the visible spectrum, it is appreciated that a structural color will result. That is to say the wrinkles 10 a will cause a color to be perceived by altering the way light travels at different dimensions, as opposed to chemical colors that rely on the absorption of certain wavelength lights by pigment molecules. It is appreciated that the colors are highly angle dependent; that is to say, the viewing angle contributes to the actual color perceived. It is further appreciated that this process presents advantages over conventional structural color forming techniques that require the use of lithographic templates, which can be relatively expensive and require dedicated equipment not widely accessible.

Through the creation of structural colors, arbitrary images can be captured and displayed using wrinkle based diffraction colors. For example, where the relief 18 a presents a protruded logo or indicia, the letters can be made to appear to protrude out of the surface 12 (i.e., three-dimensionally), while in fact the surface 12 is macroscopically smooth. This illusion results because the edge of the letters is colored to resemble shading even though no pigment is introduced in the process. By using a recessed relief (FIGS. 2 a-c), an engaging surface or logo may be produced with the letters colored, instead of the edges of the letters. With this change, the transition strain area 14 b resides in the letter or image face.

As shown in the circularly distributed wrinkles of FIG. 4, it is appreciated that the wavelength increases with the radial distance from the center of the indent. In certain instances, cracks 20 may also be produced in the wrinkle structures 10 a (FIG. 4). The existence of cracks corresponds to surpassing a critical strain above which cracks are formed. Thus, it is also appreciated that the present invention can be used as a convenient method of measuring the cracking strain/stress of a nanoscopicallythin film 16, which could otherwise be challenging using conventional methods.

This invention has been described with reference to exemplary embodiments; it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A method of forming localized wrinkles upon a surface, said method comprising: a. indenting a substrate at least partly formed of shape memory polymer, so that the substrate defines a first area experiencing purely compressive strain and a second area adjacent the first area and experiencing compressive and tensile strains; b. attaching a relaxed overlay to the substrate, so as to overlay the first and second areas; and define the surface; c. activating the polymer, so as to recover the strains; d. causing wrinkles to form in the overlay, as a result of recovering the tensile strain in the second area; and e. deactivating the polymer.
 2. The method as claimed in claim 1, wherein steps a) and b) further comprises heating the polymer to a first temperature, and applying a load to the substrate after the substrate achieves the first temperature, and cooling the polymer to a second temperature below the first temperature, while continuing to apply the load, so as to lock in the strains.
 3. The method as claimed in claim 1, wherein step a) further comprises the steps of applying a press to the substrate, wherein the press defines an engaging relief.
 4. The method as claimed in claim 3, wherein the relief presents and the first or second area defines, indicia, a logo, an engaging surface, or a shape.
 5. The method as claimed in claim 1, wherein the overlay, substrate, and strains are cooperatively configured to produce wrinkles having a wavelength within the visible spectrum.
 6. The method as claimed in claim 1, wherein the polymer consists essentially of an aromatic diepoxide, an aliphatic diepoxide, and an aliphatic diamine curing agent.
 7. The method as claimed in claim 1, wherein the overlay is a metallic thin film.
 8. The method as claimed in claim 7, wherein the overlay is formed of white gold.
 9. The method as claimed in claim 1, wherein step c) further includes the steps of depositing the overlay.
 10. The method as claimed in claim 1, wherein the substrate presents a first modulus, the overlay presents a second modulus greater than the first, and the first and second modulus present a ratio greater than a predetermined threshold ratio.
 11. The method as claimed in claim 10, wherein the overlay presents a critical bucking strain (ε_(c)), and steps a) and c) further includes the steps of defining and recovering a tensile strain greater than the critical bucking strain.
 12. The method as claimed in claim 11, wherein the critical bucking strain is determined in accordance with the formula: ε_(c) =[E _(s) ² /E _(f) ²]^(1/3) E_(s) is the modulus of the substrate, and E_(f) is the modulus of the film.
 13. The method as claimed in claim 1, wherein the second area circumscribes the first area.
 14. The method as claimed in claim 1, wherein the overlay presents a thickness approximately equal to 10 nm.
 15. A method of measuring the cracking strain/stress of a nanoscopically thin overlay, said method comprising the steps of: a. indenting a substrate at least partly formed of shape memory polymer presenting a glass transition temperature, so that the substrate defines a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing compressive and tensile strains; b. attaching the overlay to the substrate, so as to cover the first and second areas, and define the surface; c. activating the polymer, so as to recover the strains; d. determining the formation of a crack within the overlay, as a result of recovering the tensile strain in the second area; and e. observing and measuring the crack relative to the strains, when the crack is formed.
 16. An assembly for forming localized wrinkles within a continuous surface, said assembly comprising: a substrate at least partially formed of a shape memory polymer presenting a glass transition temperature and first elastic modulus when activated, being plastically indented so as to define a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing compressive and tensile strains, when the polymer is deactivated, and operable to recover a the strains when the polymer is activated; and a relaxed overlay fixedly attached to and configured to cover the first and second areas, and defining the surface, a second elastic modulus, and a height, wherein the second elastic modulus is greater than the first modulus, and the moduli, height, and strains are cooperatively configured to cause buckling in the overlay, when the polymer is activated and recovers the strains.
 17. The method as claimed in claim 16, wherein the polymer consists essentially of an aromatic diepoxide, an aliphatic diepoxide, and an aliphatic diamine curing agent.
 18. The method as claimed in claim 16, wherein the overlay is a metallic thin film.
 19. The method as claimed in claim 18, wherein the overlay is formed of white gold. 